AIPMT PRELIMS 2004 Chemistry Aromatic Compounds MCQ Question

To understand the question regarding the diazotization of aniline and its reaction with dimethyl aniline, let's break down the process and analyze the provided options.

Step 1: Understanding Diazotization

Aniline, which has the formula $\text{C}_6\text{H}_5\text{NH}_2$, undergoes diazotization when treated with nitrous acid ($\text{HNO}_2$) at low temperatures. This process converts the amino group ($-NH_2$) into a diazonium group ($-N_2^+$), yielding benzenediazonium chloride ($\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$).

Step 2: Reaction with Dimethyl Aniline

Dimethyl aniline, which is $\text{C}_6\text{H}_4(\text{N(CH}_3)_2)\text{NH}_2$, contains two methyl groups attached to the nitrogen atom. This compound is a stronger nucleophile compared to aniline due to the electron-donating effect of the methyl groups.

When benzenediazonium chloride is treated with dimethyl aniline, an electrophilic aromatic substitution occurs, resulting in the formation of a colored azo compound. The structure of this resulting compound can be derived from the reaction mechanism.

Analyzing the Correct Answer

The correct answer is:

B) $(\text{CH}_3)_2\text{N} - \text{N} = \text{N}$

This is the structure of the product formed after the reaction between the diazonium ion and dimethyl aniline. The azo linkage ($-N=N-$) indicates that an electrophilic attack has occurred, and the nitrogen from the diazonium has bonded to the dimethyl aniline, which retains the two methyl groups.

Why Other Options are Incorrect

Let's examine the other options:

• A) $\text{CH}_3 - \text{N} = \text{N} - \text{NH}_2$: This structure suggests that there is a methyl group directly attached to a nitrogen atom that is part of the diazonium compound. However, it does not represent the product formed from the reaction of dimethyl aniline with the diazonium compound.

• C) $(\text{CH}_3)_2\text{N} - \text{N} - \text{H}$: This structure indicates that there is a hydrogen atom instead of another nitrogen atom attached to the end of the azo group. This does not accurately represent the product since the reaction should yield a compound with a diazo bond.

• D) $\text{CH}_3\text{NH} - \text{N} = \text{N} - \text{NHCH}_3$: This structure implies that there is an amine group attached to the nitrogen of the diazonium, which would not happen in this reaction. The expected product involves two methyl groups from dimethyl aniline, thus making this configuration incorrect.

Conclusion

The reaction of aniline with nitrous acid followed by treatment with dimethyl aniline results in the formation of a colored azo compound with the structure $(\text{CH}_3)_2\text{N} - \text{N} = \text{N}$. This compound arises due to the substitution reaction where the diazonium ion acts as an electrophile, and dimethyl aniline acts as the nucleophile, leading to the correct answer being B.

Key Takeaway

In summary, this question illustrates the importance of understanding electrophilic aromatic substitution and the stability of the resulting azo compounds in organic chemistry. The presence of electron-donating groups (like methyl groups) enhances the nucleophilicity of the aniline derivative, facilitating the reaction and yielding colored products, which are often used in dyes and pigments.